fix: include api plugin in server install alongside admin2

Both api and admin2 are bundled with the Grav 2.0 zip and not available
via GPM. Extract and install both during remote-install. Remove the
ad-hoc remote-install-admin2 target — the main install now covers it.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Vgmzx8VTTTmCskSpQtsLTr

Makefile

@@ -110,6 +110,7 @@ remote-fetch:
 remote-install-plugins:
 	$(SSH) "cd $(WEBROOT) && php bin/gpm install $(shell cat plugins.txt | tr '\n' ' ') -y"
 
+
 remote-upgrade-grav:
 	$(SSH) "cd $(WEBROOT) && php bin/grav upgrade"